This invention relates generally to video displays and television picture displays utilizing the cathode ray tube, and is particularly concerned with establishing and maintaining image quality in such displays.
Cathode ray tubes used in alphanumeric displays typically have a glass face panel with a cathodoluminescent screen on the inner surface thereof. Typically, numbers, letters, characters or symbols are generated in alphanumeric displays and presented on the screen in a very precise dot matrix, or "character block" format. For example, a five dot by seven dot character is generated within a seven dot by nine dot character block. In the seven-by-nine matrix, there are 63 discrete dots available for forming each character. For maximum image discernibility, the cathode ray tube forming the image is typically driven to provide very high contrast. The precise timing of the scanning circuits ensures that the discrete dots are located at the exact same points on the screen.
The cathode ray tube screen has a phosphor constituent vulnerable to patterned discoloring known as "character burn." Character burn is caused by static area electron bombardment of such time and intensity as to discolor the phosphor, but not appreciable degrade phosphor efficiency. For example, the repeated exciting of the phosphor at the same location within the respective character blocks can result in character burn. This discoloring is manifested as a browning of the screen of monochromatic tubes, for example, with a browning in the localized area varying in darkness depending upon the frequency of use of the particular character. For example, those character blocks which experience more use than others, such as the characters e, t, o, i and n, will show darker discoloration than others which appear less often. Character burn may appear very quickly; e.g., after a few hours of use.
The effect of character burn is, in essence, a cosmetic one in which the screen exhibits an undesired mottling which can result in customer complaints.
The type of discoloring resulting from character burn is normally not visible on the screen of a standard television picture display, in which all areas of the screen are scanned relatively uniformly. However, for example, if the television display is used for prolonged period of time with a video game having stationary boundary lines, the lines may become perceptible as a patterned brown discoloring of the screen.
Character burn, as it will be referred to hereafter, is distinguished from "phosphor burn," in that phosphor burn is considered to be an irreversible condition resulting from over-excitation of the phospor due to a very high beam current. High beam current can be the result of a fault in the voltage control circuit of the high voltage power supply. The effect is a deterioration in phosphor efficiency to such an extent that there is little or no light output from the burned areas when the screen is excited.
The prior art is directed primarily to means for causing the image on the cathode ray tube face plate to move imperceptibly and thus prevent phosphor burn. Henderson in U.S. Pat. No. 4,127,796 set forth means for superimposing or adding to the beam deflecting voltages an additional triangular waveform. The combined effect of the offset waveforms is said to be such as to constantly move the entire picture tube electron scan patttern over an infrequently recurring path at a slow enough rate to be imperceptible to the viewer. Rudert in U.S. Pat. No. 3,255,309 discloses means operable to produce a periodic deflection of electrons emitted from a camera tube photo-cathode, with the periodicity of the deflection being long in relation to the periodicity of field deflection. Wilcox, in U.S. Pat. No. 3,335,220 discloses a camera tube target protection system employing a varying raster size to prevent phosphor burn.
In U.S. Pat. No. 4,198,661, Gatten et al discloses electrical circuits for preventing burn spots in the CRT phosphor. In one embodiment of the invention, the absolute value of the rate of change of deflection of the beam as measured by horizontal and vertical yoke current is divided by the brightness level of the beam to produce a signal which is compared to a reference value. If the dividend exceeds the reference value, the cathode ray tube is shut down by blanking the CRT beam.